1. Field of the Invention
The present invention relates to a liquid crystal device of the pi-cell type. Such a device is suitable for use, for example, in transmissive and reflective flat panel displays, head-mounted displays, field-sequential colour displays, projection systems and three-dimensional image display systems.
2. Description of the Related Art
P. D. Berezin, L. M. Blinov, I. N. Kompanets and V. V. Nikitin xe2x80x98Electro-optic Switching in Oriented Liquid-Crystal Filmsxe2x80x99 July-August 1973 Sov. J. Quant. Electron Vol 3 pp 78-79 disclose a liquid crystal device of nematic type which is capable of achieving fast response times. The device comprises a non-twisted cell of low surface tilt, but it is not clear whether parallel or anti-parallel surface alignment directions are provided. Optical modulation is achieved mainly by re-orientation of the liquid crystal molecules near the surface regions whereas the orientation in the bulk of the material remains substantially homeotropic.
P. J. Bos and K. R. Koehler/Beran xe2x80x98The pi-Cell: A Fast Liquid Crystal Optical Switching Devicexe2x80x99 1984 Mol. Cryst. Liq. Cryst. Vol 113 pp 329-339 provide the first known disclosure of a pi-cell which, as is well known, comprises first and second alignment layers arranged to induce parallel low pre-tilt alignment in a nematic liquid crystal material disposed therebetween. The pi-cell is an example of a surface mode device in which the optical modulation is obtained mainly by reorientation of liquid crystal molecules near the surface regions, as described above. The pi-cell disclosed in this paper is substantially symmetrical in that the pre-tilt angles induced by the alignment layers are of substantially equal magnitude.
FIG. 1 of the accompanying drawings illustrates the various states of a conventional pi-cell. In the absence of an applied electric field across the liquid crystal layer, the cell is in a splay state, referred to conventionally and hereinafter as the H-state. The HS state for zero applied field and with symmetrical pre-tilt angles is illustrated at 1 with the liquid crystal directors being indicated by the lines such as 2.
As the voltage across the layer (and hence the applied electric field) is increased, the H-state becomes asymmetrical for relatively small voltages as illustrated by the HA-state at 3.
The H-states of a pi-cell do not have desirable optical properties for use in optical devices such as flat panel displays. Above a certain voltage, however, the pi-cell exhibits an alternative state known as a bend state (conventionally known and referred to hereinafter as the V-state) as shown at 4 in FIG. 1 which has more useful optical properties. In the V-state, the liquid crystal molecules have relatively low tilts in the surface regions but have a homeotropic alignment in the bulk of the material with the director substantially perpendicular to the cell surfaces. Once the V-state has been established, optical modulation is performed mainly by reorientation of the liquid crystal molecules in the surface regions with the molecules in the bulk of the layer being substantially unaffected by the variations in applied voltage within the operating range of the device.
For pi-cells of the type representative of the prior art, with a typical pretilt of for example 5xc2x0, there is a xe2x80x9coriticalxe2x80x9d or threshold voltage UV/H above which the energy of the V-state is lower than the energy of the H-state. The liquid crystal in such a pi-cell will therefore prefer to align in the V-state above UV/H. The transition from the undesirable low voltage H-state to the desired V-state is, however, non trivial and a so-called xe2x80x9cnucleationxe2x80x9d process must occur which involves the creation and movement of defects in the liquid crystal. The process of nucleating a pi-cell from an H-state to a V-state is typically rather slow, taking some seconds In typical devices.
Besides the V-state and the H-state, a pi-cell may also exhibit a twist state (conventionally know and referred to hereinafter as the T-state) as shown at 5 in FIG. 1 in which the director performs a (xc2x1)180xc2x0 twist between the alignment layers. For a typical pi-cell representative of the prior art with, for example, a typical pretilt of 5xc2x0, if the liquid crystal is in the V-state and the voltage U is lowered, there is a threshold voltage UV/T below which the T-state becomes of lower energy than the V-state. Below this threshold voltage, the liquid crystal therefore undergoes a transition from the V-state to the T-state. This transition does not involve nucleation and may proceed fairly rapidly (in typically 10""s or 100""s of milliseconds). The T-state has less desirable optical properties (such as viewing angle performance and contrast ratio) than the V-state. As the voltage U is lowered towards zero volts, the H-state will reform. However, as with the H/V transition, the H/T transition involves the nucleation of defects and is typically rather slow (of the order of seconds). Thus the T-state may exist at low voltages for some seconds before it Is replaced by the H-state.
FIG. 1 summarises the behaviour of a conventional pi-cell as the applied voltage U is first increased to a maximum U greater than  greater than UV/H and then reduced towards zero. A conventional pi-cell, which is representative of the prior art shows three main types of liquid crystal orientation: H-states, a V-state and T-states. At zero volts, the energy of the H-state is lowest, the energy of the V-state is highest and the energy of the T-state is intermediate between the energies of these other states.
The paper by Bos et al describes two modes of operating the pi-cell. In both modes, one state of the pi-cell is achieved at a relatively high voltage where the V-State is stable (its energy is lower than the energies of the H-state and the T-state.) In this operating state, the pi-cell provides a minimum of optical retardation.
The first mode applies to relatively thin cells in which the liquid crystal material is allowed to relax from the relatively high voltage V-state to a zero volt state, at which the pi-cell provides a half wave of retardation. The zero volt state is a substantially co-planar state and is achieved dynamically for in excess of 20 milliseconds (although this state may be achieved for substantially less time at higher temperatures). This state is unstable and, if it is allowed to prevail for too long, the T-state begins to form and this can then lead to nucleation of the H-state, after which re-nucleation of the V-state has to be performed in order for the pi-cell to function again. This voltage addressing scheme therefore applies a voltage U less than UV/T to a pi-cell which, at zero volts, has a lowest energy H-state, a highest energy V-state and an intermediate energy T-state and makes use of the dynamic V-state which survices in excess of 20 msec before the T-state forms (although, again, at higher temperatures this may be substantially less).
For thicker cells the second mode of operation is used in which the half wave retardation condition is reached before the onset of any significant relaxation to the T-state. A small voltage is maintained across the liquid crystal layer to hold the cell at the half wave retardation condition.
H. Nakamua xe2x80x98Dynamic Bend Mode in a Pi-Cellxe2x80x99 Dec. 1-3 1999 SID Proceedings of the 6th International Display Workshop, pp 37-40 discloses a technique of xe2x80x9cUnder-Drivingxe2x80x9d a pi-cell and refers to this as a xe2x80x9cDynamic Bend Modexe2x80x9d. This driving mode is equivalent to the first driving mode described by Bos et al with the dynamic V-state having a lifetime which increases with increasing bias voltage. There is also disclosed the use of a relatively high voltage blanking pulse during each frame in order to avoid the need to reform the V-state.
U.S. Pat. No. 4,566,758 discloses a pi-cell in which the liquid crystal material Is doped with a chiral dopant such that the ratio of the thickness of the liquid crystal layer to the chiral pitch is greater than 0.25. This type of device remains In a T-state throughout the operating voltage range with the T-state having similar optical properties to the V-state at relatively high operating voltages. Such an arrangement overcomes the problems of nucleation in conventional pi-cells but retains similar optical characteristics at relatively high operating voltages. However, at lower voltages, the effect of the inherent twist on the optical characteristics gives a poorer performance than for the conventional pi-cell in that a reduced viewing angle performance and poorer response speed are exhibited.
E. J. Acosta, M. J. Towlerand H. G. Walton xe2x80x9cThe Role of Surface Tilt in the Operation of Pi-Cell Liquid Crystal Devicesxe2x80x9d July 2000 Liquid Crystals vol 27 pp 977-984 discloses the role of surface pretilt in the operation of a pi-cell. For zero applied voltage, the H-state is stable, for a typical nematic liquid crystal material, over a range of pretilts from 0xc2x0 to about 48xc2x0 whereas the V-state becomes stable for pretilts above about 48xc2x0.
According to the present invention, there is provided a pi-cell liquid crystal device comprising; a layer of nematic liquid crystal material disposed between first and second alignment layers, which induce a pretilt in the adjacent liquid crystal material such that, for zero applied electric field, the energy of the H-state is less than the energy of each of the V-state and the T-state and the energy of the V-state is less than or equal to the energy of the T-state; and a drive arrangement for selectively applying to at least one region of the layer a first electric field, at which the energy of the V-state is less than the energy of each of the H-state and the T-state, and a second electric field, which is of smaller magnitude than the first electric field and at which the energy of the H-state is less than the energy of each of the V-state and the T-state.
The energy of V-state may be less than the energy of the T-state for zero applied field.
The second electric field may have a substantially zero magnitude.
The first and second electric fields may select first and second extreme optical states of the optical range of the at least one region. The first and second extreme optical states may comprise first and second retardations of the at least one region which differ from each other by an odd number of half wavelengths of optical radiation for which the device is intended. The first and second retardations may differ by half a wavelength. The first and second extreme optical states may comprise maximum and minimum attenuation, respectively. As ah alternative, the first and second extreme optical states may comprise minimum and maximum attenuation, respectively.
The pretilt may be less than substantially 50xc2x0 and may be less than substantially 48xc2x0.
The pretilt may be greater than substantially 20xc2x0 may be greater than or equal to substantially 26xc2x0 and may be greater than or equal to substantially 29xc2x0.
The liquid crystal material may have elastic constants K11, K22, K23, each of which is less than 50 pN at room temperature. Each of the elastic constants may be less than 30 pN throughout the operating temperature range of the device.
The liquid crystal material may have a dielectric constant greater than substantially two.
The liquid crystal material may have a dielectric constant less than substantially 15. The dielectric constant may be less than 10.
It has been unexpectedly found that there exists a critical range of pretilt angles (where this range is outside that commonly used in known pi-cells), which results in the energy of the T-state being highest at zero volts, the energy if the H-state being lowest and the energy of the V-state being intermediate. This may be used to contruct pi-cells for which it is never the case that it is energetically preferable for the T-state to form at any voltage. Such a pi-cell may have advantageous optical properties when used with specific types of voltage addressing scheme.
It is possible to provide a pi-cell liquid crystal device which is operated in such a way that the T-state is never the lowest energy state and is thus never stable. Thus, twist does not occur in the device, which is capable of operating at fast switching speeds and with a good degree of optical modulation. Since the temperature and time dependant twist state never forms, good viewing angle performance as a function of temperature can be achieved by using optical compensating films.
Relatively fast initial growth of the nucleated V-state from the initial H-state is achieved. The device cannot relax into the T-state, even for zero applied voltage, and relaxation into the U-state takes a relatively long time compared with the time taken to modify the liquid crystal orientation whilst the pi-cell remains in the V-state. Thus, the problems associated with nucleation and renucleation in the operation of conventional pi-cells are substantially reduced. An additional advantage of more flexibility In the design of addressing waveforms than in the paper by Nakamura mentioned hereinbefore follows from this.